Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g. AlGaAs) have been developed since the mid-80's (Kinoshita et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June 1987). They have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years (Choquette et al., Proceedings of the IEEE, Vol. 85, No. 11, November 1997). With the success of these near-infrared lasers, attention in recent years has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (Wilmsen, Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001). There are many potential applications for visible lasers, such as, display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (Ishigure et al., Electronics Letters, 16th March 1995, Vol. 31, No. 6). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes (either edge emitters or VCSELs) that produce light output that spans the visible spectrum.
In an effort to produce visible wavelength VCSELs it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials can enjoy a number of advantages over inorganic-based gain materials in the visible spectrum. For example, typical organic-based gain materials have the properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers are relatively inexpensive to manufacture, can be made to emit over the entire visible range, can be scaled to arbitrary size and, most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip. Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The laser gain material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as, microcavity (Kozlov et al., U.S. Pat. No. 6,160,828, issued Dec. 12, 2000), waveguide, ring microlasers, and distributed feedback (see also, for instance, Kranzelbinder et al., Rep. Prog. Phys. 63, (2000) 729-762 and Diaz-Garcia et al., U.S. Pat. No. 5,881,083, issued Mar. 9, 1999). A problem with all of these structures is that in order to achieve lasing it was necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities since this generally results in more compact and easier to modulate structures.
A main barrier to achieving electrically pumped organic lasers is the small carrier mobility of organic material, which is typically on the order of 10−5 cm2/(V−s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (Kozlov et al., Journal of Applied Physics, Volume 84, No. 8, Oct. 15, 1998). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales as the carrier mobility), low carrier mobilities result in orders of magnitude having more charge carriers than singlet excitons; one of the consequences of this is that charge-induced (polaron) absorption can become a significant loss mechanism (Tessler et al., Applied Physics Letters, Volume 74, Number 19, May 10, 1999). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of ˜100 W/cm2 (Berggren et al., Letters to Nature, Volume 389, page 466, Oct. 2, 1997), and ignoring the above mentioned loss mechanisms, would put a lower limit on the electrically-pumped lasing threshold of 1000 A/cm2. Including these loss mechanisms would place the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, which can be supported by organic devices (Tessler, Advanced Materials, 1998, 10, No. 1, page 64).
One way to avoid these difficulties is to use crystalline organic material instead of amorphous organic material as the lasing media. This approach was recently taken (Schon, Science, Volume 289, Jul. 28, 2000) where a Fabry-Perot resonator was constructed using single crystal tetracene as the gain material. By using crystalline tetracene, larger current densities can be obtained, thicker layers can be employed (since the carrier mobilities are on the order of 2 cm2/(V−s)), and polaron absorption is much lower. Using crystal tetracene as the gain material, resulted in room temperature laser threshold current densities of approximately 1500 A/cm2.
An alternative to electrical pumping for organic lasers is optical pumping by incoherent light sources, such as, light emitting diodes (LEDs), either inorganic (McGehee et al., Applied Physics Letters, Volume 72, Number 13, Mar. 30, 1998) or organic (Berggren et al., U.S. Pat. No. 5,881,089, issued Mar. 9, 1999). This possibility is the result of unpumped organic laser systems having greatly reduced combined scattering and absorption losses (˜0.5 cm−1) at the lasing wavelength, especially when one employs a host-dopant combination as the active media. Even taking advantage of these small losses, the smallest reported optically pumped threshold for organic lasers to date is 100 W/cm2 based on a waveguide laser design (Berggren et al., Letters to Nature Volume 389, Oct. 2, 1997). Since off-the-shelf inorganic LEDs can only provide up to ˜20 W/cm2 of power density, it is necessary to take a different route to avail of optically pumping by incoherent sources. Additionally, in order to lower the lasing threshold it is necessary to choose a laser structure that minimizes the gain volume; a VCSEL-based microcavity laser satisfies this criterion. Using VCSEL-based organic laser cavities should enable optically pumped power density thresholds below 5 W/cm2. As a result practical organic laser devices can be driven by optically pumping with a variety of readily available, incoherent light sources, such as LEDs.
There are a few disadvantages to organic-based gain media, but with careful laser system design these can be overcome. Organic materials can suffer from low optical and thermal damage thresholds. Devices will have a limited pump power density in order to preclude irreversible damage to the device. Organic materials additionally are sensitive to a variety of environmental factors, like oxygen and water vapor. Efforts to reduce sensitivity to these variables typically result in increased device lifetime.
One of the advantages of organic-based lasers is that since the gain material is typically amorphous, devices can be formed inexpensively when compared to lasers with gain materials that require a high degree of crystallinity (either inorganic or organic materials). Additionally, lasers based upon organic amorphous gain materials can be fabricated over large areas without regard to producing large regions of single crystalline material; as a result they can be scaled to arbitrary size resulting in greater output powers. Because of their amorphous nature, organic-based lasers can be grown on a wide variety of substrates; thus, materials such as glass, flexible plastics, and Si are possible supports for these devices. Thus, there can be significant cost advantages as well as a greater choice in usable support materials for amorphous organic-based lasers.